In radio communication, control data and speech data are encrypted and transmitted for the improvement of security performance. Radio communication used by a general person such as a cordless telephone widely adopts a common key cryptosystem in which the same key is used for both encryption and decryption. In the radio communication of the cordless telephone, for improved convenience, the key to be used in the common key cryptosystem is automatically generated, and the common key is shared between the base unit and the handset.